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Two-proton Radioactivity: the Interesting Case of
67Kr
and Further Studies
J. Giovinazzo, T. Goigoux, B. Blank, P. Ascher, M. Gerbaux, S. Grévy, T.
Kurtukian-Nieto, C. Magron, P. Doornenbal, N. Fukuda, et al.
To cite this version:
J. Giovinazzo, T. Goigoux, B. Blank, P. Ascher, M. Gerbaux, et al.. Two-proton Radioactivity: the Interesting Case of67Kr and Further Studies. 36th Mazurian Lakes Conference on Physics, Sep 2019,
TWO-PROTON RADIOACTIVITY: THE INTERESTING
CASE OF 67Kr AND FURTHER STUDIES∗
J. Giovinazzoa, T. Goigouxa, B. Blanka, P. Aschera M. Gerbauxa, S. Grévya, T. Kurtukian-Nietoa, C. Magrona P. Doornenbalb, N. Fukudab, N. Inabeb, G.G. Kissb,†, T. Kubob S. Kubonob, S. Nishimurab, H. Sakuraib, Y. Shimizub, C. Sidongb
P.-A. Söderströmb, T. Sumikamab, H. Suzukib, H. Takedab P. Vib, J. Wub, D.S. Ahnc, J. Agramuntc, A. Algorac,d
V. Guadillac, A. Montaner-Pizac, A.I. Moralesc S.E.A. Orrigoc, B. Rubioc, Y. Fujitae, M. Tanakae W. Gelletlyc,f, P. Aguilerag, F. Molinag, F. Dielh, D. Lubosi G. de Angelisj, D. Napolij, C. Borceak, A. Bosol, R.B. Cakirlim
E. Ganioglum, J. Chiban, D. Nishimuran, H. Oikawan, Y. Takeio S. Yagio, K. Wimmero, G. de Francep, S. Goq, B.A. Brownr
aCentre d’Études Nucléaires de Bordeaux Gradignan, France bRIKEN Nishina Center, Wako, Saitama, Japan cInstituto de Física Corpuscular, Valencia, Spain
dMTA Atomki, Debrecen, Hungary eDepartment of Physics, Osaka University, Japan fDepartment of Physics, University of Surrey, United Kingdom
gComisión Chilena de Energía Nuclear, Santiago, Chile hInstitute of Nuclear Physics, University of Cologne, Germany iPhysik Department E12, Technische Universität München, Germany
jLaboratori Nazionali di Legnaro dell’INFN, Italy kNIPNE IFIN-HH, Bucharest-Măgurele, Romania
lINFN Sezione di Padova and Dipartimento di Fisica, Universitá di Padova, Italy mDepartment of Physics, Istanbul University, Vezneciler, Istanbul, Turkey
nDepartment of Physics, Tokyo University of Science, Japan oDepartment of Physics, University of Tokyo, Japan pGrand Accélérateur National d’Ions Lourds, Caen, France qDepartment of Physics and Astronomy, University of Tennessee, USA
rDepartment of Physics and Astronomy, NSCL, MSU, USA
(Received January 7, 2020)
∗
Presented at the XXXVI Mazurian Lakes Conference on Physics, Piaski, Poland, September 1–7, 2019.
†
Present address: Institute for Nuclear Research (ATOMKI), 4001 Debrecen, Hungary.
578 J. Giovinazzo et al.
We report on the observation of67Kr that has been produced in an ex-periment performed at the RIKEN/BigRIPS facility. The two-proton decay of67Kr has been evidenced and this nucleus is thus the fourth observed long
lived ground-state two-proton emitter, after45Fe, 48Ni and54Zn. In
addi-tion, the decay of several isotopes in the mass region has been investigated. While for previous cases of two-proton radioactivity, the theoretical mod-els could reproduce the measured data, this is not the case anymore for
67Kr. Two interpretations have been proposed to explain this discrepancy:
a transition between real two-proton and sequential decay or the influence of deformation. These hypotheses will be tested in future experiments by measuring the angular and energy correlations of the emitted protons.
DOI:10.5506/APhysPolB.51.577
1. Introduction
The two-proton radioactivity is a very exotic decay mode that occurs for nuclei beyond the proton drip-line, with an even atomic number Z. Together with 1-proton radioactivity (for odd-Z nuclei), the phenomenon was suggested in the 1960s by theoretical predictions [1]. The process has been evidenced in the decay of 45Fe in experiments at GANIL/LISE3 [2] and GSI/FRS [3] in 2002. For candidate nuclei, the last two protons are not bound by strong interaction, but the ground-state energy is below the Coulomb barrier, which gives the life-time of the emitter. In addition, due to pairing, sequential emission of the protons is energetically forbidden, and the correlated protons have to be emitted simultaneously. After the emis-sion by tunnel effect through the barrier, the sub-system (p–p, or 2He) is unbound.
Today, 4 long-lived emitters (in the ms range) are known: 45Fe [2–5],
48Ni [6,7],54Zn [8] that have been produced and studied at GSI, GANIL and
NSCL, and recently 67Kr [9–11], in an experiment at the RIKEN/BigRIPS facility that is reported in this paper. These nuclei have been produced at fragmentation facilities, and the first experiments were based on the implantation-decay correlation technique in silicon detectors. This allowed for an indirect observation of the two-proton radioactivity based on global quantities: the half-life, the branching ratio and the energy of the decay. Calculations performed in a theoretical 3-body framework [12, 13] showed that the p–p correlation pattern is sensitive to the orbital configuration of the nuclei. New devices have been developed, in order to measure the tracks of the individual particles in time projection detectors (TPC) and deduce the angular and energy correlations of the emitted protons [4,5,14].
In this paper, we report some results from the experiment performed at RIKEN to produce nuclei at the proton drip-line in the 67Kr mass region. The exotic nuclei have been produced in the fragmentation of a 78Kr beam
(250 nA, 350 A MeV) on a beryllium target, and selected by the BigRIPS and the ZDS separators [15]. They were implanted in the WAS3ABi [16] device consisting of 3 DSSSDs (each detector is 1 mm thick, with 60 vertical and 40 horizontal strips of 1 mm width). The charged particles emitted during the radioactive decay of implanted nuclei were detected in the silicon detectors that are surrounded by the EURICA [17] array of germanium detectors for the coincident γ detection.
New isotopes have been identified, and the decay of several nuclei have been investigated. In the case of67Kr, we could conclude on two-proton emis-sion, in competition with β+/electron capture decay. For previous cases of two-proton radioactivity, the experimental data could be fairly well under-stood in available theoretical frameworks. In the case of67Kr, these models could not reproduce the measured half-life, and this triggered new theoretical developments [18,19] in two-proton radioactivity modeling. Further exper-imental studies should be carried out to confirm the proposed hypotheses, by measuring p–p correlations for this decay.
2. New isotopes
The fragments were identified with the BigRIPS standard detection, from energy loss, time-of-flight and magnetic rigidity determination [11]. The result is presented in Fig. 1. 63Se, 68Kr and 67Kr have been observed for the first time [9], with respectively 348, 477 and 82 counts at the F7 focal plane of the separator. Only a fraction of these nuclei is transmitted through the ZDS spectrometer to the implantation/decay detectors. This fraction is about 40% for the most exotic fragments.
Fig. 1. Fragment identification matrix from the BigRIPS detection. The circles indicate the isotopes that have been observed for the first time.
580 J. Giovinazzo et al.
3. Beta-decay results
For ions implanted in the WAS3ABi detectors, the correlation of subse-quent decay events allows to determine the charged particles energy distribu-tion (with coincident γ rays) and the time distribudistribu-tion between implantadistribu-tion and decay events from which the decay half-life is estimated. The details of the analysis can be found in Ref. [11].
TABLE I Comparison of the half-lives and total beta-delayed proton emission ratios measured for the decay of several isotopes produced in the experiment. The results are taken from Ref. [11].
Isotope Isospin Half-life T1/2 [ms] Total β-proton
TZ branching [%]
Previous Present Previous Present
63Se −5/2 13.2 ± 3.9 89 ± 11 59Ge 13.3 ± 1.7 93 ± 7 68Kr −2 21.6 ± 3.3 89+11 −10 69Kr −3/2 28 ± 1 [20] 27.8 ± 1.6 93+7 −6 27 ± 3 [21] 99+1−11 [21] 65Se 33 ± 4 [21] 34.2 ± 0.7 88+12 −13 [21] 61Ge 45 ± 6 [22] 40.7 ± 0.4 62 ± 4 [22] 87 ± 3 40 ± 15 [23] 57Zn 48 ± 3 [22] 45.7 ± 0.6 78 ± 17 [22] 40 ± 10 [24] 55Cu 57 ± 3 [25] 55.5 ± 1.8 0 [25] 27 ± 8 [26] 15.0 ± 4.3 [26] 64As −1 72 ± 6 [27] 69.0 ± 1.4 18+43−7 [28] 60Ga 76 ± 3 [29] 70.8 ± 2.0 70 ± 13 [28] 70 ± 15 [30] 1.6 ± 0.7 [30] 56Cu 80 ± 2 [29] 80.2 ± 0.7 82 ± 9 [28] 93 ± 3 [31] 0.40 ± 0.12 [31] 78 ± 15 [32] 65As −1/2 126 ± 5 [27] 130.3 ± 0.6 128 ± 16 [33] 63Ge 156 ± 11 [29] 153.6 ± 1.1 149 ± 4 [27] 150 ± 9 [34]
The decay of59Ge,63Se and67,68Kr has been measured for the first time.
59Ge and63Se were candidates for two-proton radioactivity, but no evidence
of such a decay branch could be observed in the data. In addition, the decay of several β-delayed proton or β emitters has been analyzed. The results are summarized in Table I.
When previous measurements are available, the half-lives are all in good agreement with literature values. For the only proton emission branching ratio that was previously measured (61Ge decay), our value is 5 standard deviations away from the previous value. The reason for this discrepancy is that in previous measurement, the proton emission at an energy below the main peak at 3.2 MeV is not observed, resulting in an underestimated branching ratio. When considering only the proton emission from this peak, both measurements are in good agreement.
4. Two-proton radioactivity of 67Kr
The charged particle energy and time distributions for the decay of67Kr are presented in Fig.2. The peak at 1690 keV is assigned to the two-proton emission branch (details in Ref. [10]). We obtained a half-life of 7.4 ± 2.9 ms for the nucleus and a 2P branching ratio of 37 ± 14%. The partial half-life is thus 10 ± 6 ms for the β decay, in agreement with the value from the gross theory [35], and 20 ± 11 ms for two-proton emission.
The two-proton half-life has been compared to a hybrid model [36] that combines the good dynamics of a 3-body model [37, 38] with the partial emission spectroscopic amplitudes from the configuration interaction model (details in Ref. [36]). While the calculated half-lives are in fairly good agree-ment (within a factor of 2) with the experiagree-mental values for the previously studied precursors (see also Refs. [14, 39]), in the case of 67Kr, the differ-ence is in the order of a factor 20. This result has triggered new theoretical calculations aiming to explain this large discrepancy.
A first hypothesis is related to the decay mechanism. According to a recent semi-analytical R-matrix calculation [18], depending on the position of the 66Br+p resonance, the decay could take place in a transition region between a direct and a sequential two-proton emission. In such a case, the energy sharing between the emitted particles must be sensitive to the sequential component, with a contribution that is not centered on an equal sharing of the energy.
Another possible reason of the discrepancy is that67Kr is most likely lo-cated in a deformation region [40]. A calculation in the three-body Gamow coupled-channel (GCC) framework resulted in a half-life of 24+10−7 ms [19], in agreement with the experiment, when considering a deformation of β2 '
−0.3 due to the g9/2 orbital. The GCC calculation also predicts proton– proton angular correlations with different distributions whether the g9/2 or-bital is considered or not.
582 J. Giovinazzo et al.
Fig. 2. (Color online) The upper plot shows the charged particle energy distribution in the decay of 67Kr, measured in the detector where the implantation occurred. The black/blue histogram corresponds to the events with no coincident signal from a β particle in neighboring detectors. The peak at 1690 keV corresponds to the two-proton transition. The lower plot represents the time distribution of events after implantation. The fit (gray/red curve), taking into account the daughter decay of the different decay branches (two-proton and β), gives a half-life of 7.4 ± 2.9 ms.
5. Further studies
The experimental perspectives for ground-state two-proton radioactivity studies follow two main directions. The first one is to improve the knowl-edge on already identified emitters such as 48Ni, 54Zn and 67Kr for which
very little or nothing is known about the correlations of emitted protons. The second direction is to search for new candidates, which implies to pro-duce and identify new isotopes and observe their decay modes. The proton drip-line between the known cases and the tin isotopes is of special interest because if other emitters are found, this would map a region between the closed shells Z = 28 and Z = 50, crossing a deformation zone. This region of the table of isotopes should be reachable in a near future with the SuperFRS at GSI/FAIR.
The GCC calculation is performed for67Kr, but also for the spherical case
48Ni as a benchmark for the model. In both cases, the correlation pattern is
not measured, but this can be achieved using ACTAR TPC [41] as tracking detector, in order to measure the angle and energy of individual protons. This device has been used in a recent experiment [42] at GANIL/LISE3 where the proton radioactivity of isomers in 53Co and 54Ni has been suc-cessfully observed. An experimental program for the correlation measure-ment is proposed at GANIL/LISE3 for 48Ni [43] and at RIKEN/BigRIPS for67Kr [44].
In the decay of67Kr, the angular distribution will allow to discriminate between GCC predictions with or without the g9/2 orbital, but also to test the sequential decay hypothesis by measuring the energy sharing of the pro-tons. It is also necessary to measure the angular correlations of the protons in the decay of48Ni because the predictions from the GCC model differ from the one that we can extrapolate from the 3-body model, available only for
45Fe [4,12] and for54Zn [14].
On the experimental side, in addition to the measurements of the proton– proton correlation pattern that will be performed for48Ni and67Kr, a higher statistics would also be of great interest in the case of54Zn for a detailed com-parison with available calculations. With the development of intense beams and new facilities (such as FAIR/SuperFRS), the search for new emitters should allow to reach the next closed shell at Z = 50.
6. Conclusion
During the experiment performed at the RIKEN/BigRIPS facility, we produced nuclei at the proton drip-line in the region Z ≤ 36 and stud-ied their radioactive decay. From the analysis of the 67Kr decay data, we measured a 7.4 ± 2.9 ms half-life with a two-proton radioactivity branch of 37 ± 14%. The experimental results are in disagreement with expectation from available calculations. This lead to new theoretical works suggest-ing either a transitional situation between direct and sequential two-proton emission or the influence of deformation in order to reproduce the measured half-life. These hypotheses will be addressed in further experiments foreseen to be carried out with the tracking detector ACTAR TPC to measure the angular correlation and the energy sharing of the emitted protons.
This experiment was carried out under programs No. NP0702-RIBF4R1-01, No. NP1112-RIBF82, and No. NP1112-RIBF93 at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. We would like to thank the whole RIBF accelerator staff for the support during the experiment and for maintaining excellent beam conditions with record
in-584 J. Giovinazzo et al.
tensities. This work was supported by the French–Japanese “Laboratoire international associé” FJ-NSP, by JSPS KAKENHI grant No. 25247045, by the Istanbul University Scientific Project Unit under project No. BYP-53195, by the Max-Planck Partner group, by the NSF grant No. PHY-1404442, by the UK Science and Technology Facilities Council (STFC) grant No. ST/F012012/1, by the Japanese Society for the Promotion of Science under grant No. 26 04808, by MEXT Japan under grant No. 15K05104, and by the Spanish Ministerio de Economia y Competitividad under grants No. FPA2011- 24553 and No. FPA2014-52823-C2-1-P, Centro de Excelencia Severo Ochoa del IFIC SEV-2014-0398; Junta para la Ampliación de Estu-dios Programme (CSIC JAE-Doc contract) cofinanced by FSE. This work was supported by NKFIH (NN128072), by the New National Excellence Pro-gram of the Ministry for Innovation and Technology (UNKP-19-4-DE-65). G.G. Kiss acknowledges support form the Janos Bolyai research fellowship of the Hungarian Academy of Sciences.
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